Why wouldn't you buy this.
Cod4 is by far the best release in the entire Cod series of games. The campaign mode and multiplayer are 'still' vastly superior to all recent efforts including World at War, Modern Warfare 2 and Black Ops.
Play the campaign on Veteran for a proper challenge and you won't have any complaints about it being too short. There are difficulty spikes, which will make you want to break something. There's unforgiveable grenade spamming by the enemy, but it's still a joy to play. The pace and variety of the campaign is exceptional. It flows perfectly and doesn't descend into the ridiculous like MW2 i.e. you don't go from fighting in the dessert to back yards in America. It all feels more realistic (for a computer game).
The characters in the campaign are fantastic and you will become attached to them. Very rare these days. This is all married with excellent voice acting and a great sense of humour delivered by your SAS comrades.
Playing Cod4 to experience the legendary "All Ghillied Up" sniping level is worth the purchase price alone. It's been poorly copied and never bettered since. No spoilers here.
The multiplayer is a lesson in balance, which was completely missing from MW2. There are only 3 set kill streaks, 3 kills = UAV, 5 = mortar strike, 7 = attack helicopter. No nukes, no chopper gunner. It's perfect.
The weapons balance are excellent. Many of the same weapons featured in MW2 and were tweeked and completely ruined. Also, you get a had gun as your only secondary weapon, which again added much needed balance missing from MW2.
Nothing I can say will do this excellent game justice. Cod4 is still graphically stong. There is still a strong online community and this shows no signs of fading. Why, because it's that good.
Just buy it and enjoy.